1. Field of the Invention
The invention relates to a communication method and arrangement based on a packet switched connection in a radio system.
2. Description of the Related Art
It is vital to have power control of signals in a radio system. This is of a particular importance in a CDMA (Code Division Multiple Access) radio system, which is interference-limited. The main task of the power control in a CDMA radio system is to limit signal powers, and hence increase capacity by decreasing interference inside each cell.
For example, in a WCDMA (Wide band CDMA) radio system the power control mechanism includes an inner loop power control and an outer loop power control.
The purpose of the inner loop power control is to eliminate rapid variations in the strength of a received signal caused by the radio channel.
In the inner loop power control, a base station compares the SIR (Signal Interference Ratio) of the received signal to a target SIR. If the SIR of the received signal is below the target SIR, the base station transmits a signal commanding the user terminal to increase its transmission power. Correspondingly, if the SIR of the received signal is above the target SIR, the base station transmits a signal commanding the user terminal to decrease its transmission power.
In the outer loop control a radio network controller (RNC) compares the quality of service to a target quality. The quality can be measured as, for instance, BER (Bit Error Rate), BLER (Block Error Rate), FER (Frame Error Rate), CRC (Cyclic Redundancy Check), soft information from the decoder, ratio of received bit energy and noise, etc. If the quality of service is below the target quality, the RNC commands the base station to increase its target SIR. Similarly, if the quality of service is above the target quality, the RNC commands the base station to decrease its target SIR.
In radio systems utilizing packet-switched connection, the packets are usually protected against noise, fading and interference by channel coding, such as FEC (Forward Error correction Coding). In spite of protection, failure may occur in the reception of a packet, which can be compensated for by retransmission. The retransmission takes place when the receiving transceiver of packets requests the faulty packet to be repeated. This can be performed by an ARQ (Automatic Repeat Request) mechanism. In a receiver utilizing HARQ (Hybrid ARQ), the faulty packet and the retransmitted packet can be combined. The combining can be especially effective if different transmissions of the same packet are utilized in decoding.
There are, however, problems related to the use of retransmission with power control, especially in the case of HARQ. When a packet is communicated unsuccessfully, the outer loop power control increases the target SIR, which unnecessarily leads to a higher transmission power during retransmission of the packet. The increased transmission power in relation to interference in turn decreases the capacity and service quality of the radio system.